This invention relates to an ultrasonic Doppler imaging apparatus used for measuring the velocity of blood flow in the human body and the like on a real time basis.
There is known a method of ultrasonic pulse Doppler measurement in which ultrasonic pulse waves are emitted to a blood vessel of human a body and reflected ultrasonic waves, that have been rendered Doppler shifts in dependence on the blood flow velocity, are detected to thereby obtain information on blood flow. Also known is an ultrasonic Doppler imaging apparatus in which the ultrasonic pulse Doppler measuring method is combined with the pulse reflection method for producing a tomographic image of a diagnostic object, such as the heart, through the emission of ultrasonic pulse waves to the object body and the detection of reflected ultrasonic pulse waves from the object body, so that the blood flow information and tomographic image (B-mode image) information are obtained by means of a single ultrasonic probe and the blood flow information modulated in color graphics is superimposed on the tomographic image on a real time basis. The above-mentioned ultrasonic Doppler imaging apparatus, which is disclosed in Japanese Patent Unexamined Publication No. 57-128138, is based on the principle of operation, which will be explained in the following.
FIG. 1 shows the basic principle and the arrangement of the conventional ultrasonic Doppler imaging apparatus. Symbol Y1 denotes a blood stream, and a, b and c denote emission directions of ultrasonic pulse waves to the blood stream Y1. Indicated by 51 is a probe which emits ultrasonic pulse waves to the blood stream Y1 and receives reflected ultrasonic pulse waves from the blood stream, 52 is a reception circuit which receives a ultrasonic pulse wave signal from the probe, 53 is a quadrature phase detection circuit which receives the output signal of the reception circuit and implements quadrature phase detection for the signal, 54 and 55 are high-pass filters, 56 is a frequency analyzer which receives the output signals of the high-pass filters and implements the frequency analysis for the signals, 57 is a frame memory which stores the output of the frequency analyzer, and 58 is a display device.
Next, the operation of the foregoing conventional apparatus will be explained. In FIG. 1, when an ultrasonic pulse wave is emitted to the blood stream Y1 in a live body under test, it is dispersed by flowing blood cells and the center frequency fc is shifted by a Doppler shift to vary by fd, resulting in a received frequency of f=fc+fd. The Doppler shift frequency fd is given by the following expression (1). EQU fd=2V .multidot. cos(.theta.) .multidot. fc/c (1)
where V is the blood flow velocity, .theta. is the incident angle of the ultrasonic beam with the blood vessel, fc is the center frequency, and c is the velocity of sound.
Accordingly, the blood flow velocity V can be evaluated by detecting the Doppler shift frequency fd.
The blood flow velocity V is displayed as a two-dimensional image as follows. Initially, the ultrasonic probe 51 emits ultrasonic pulse waves in directions a, b, c, and so on sequentially toward the object body. At the beginning, ultrasonic pulse waves are emitted in the direction a several times, e.g., ten times. Each echo signal produced by Doppler shift reflection by the blood flow in the object body is received by the same probe 51, which converts the echo signal into an electric signal and delivers to the receiving circuit 52. Subsequently, the quadrature phase detection circuit 53 detects the I-channel and Q-channel Doppler shift signals as complex Doppler data. The Doppler shift signals are assessed on 256 sample points, for example, located in the emission direction of ultrasonic pulse waves.
FIG. 2 shows the arrangement of the quadrature phase detection circuit 53. In the figure, indicated by 61 is an oscillator, 62 is a shifter, 63 and 64 are mixers, and 65 and 66 are low-pass filters. A signal provided by the reception circuit 52 is fed to the mixers. The mixer 63 mixes the received signal with a signal generated by the oscillator 61, and the mixer 64 mixes the received signal with the signal provided by the oscillator and phase-shifted by 90.degree. by the shifter 62. The mixers 63 and 64 have their outputs fed through the low-pass filters 65 and 66, respectively, and the Doppler shift signals of the I and Q channels are detected.
Returning to FIG. 1, the Doppler shift signals at the same sample points, i.e., real parts and imaginary parts of ten elements of complex Doppler data resulting from ten emissions, have low-frequency component caused by the blood vessel wall and the like removed by means of the respective high-pass filters 54 and 55, and thereafter are fed to the frequency analyzer 56. The result of analysis is stored in the frame memory 57, and the images of the blood flow velocity component in the emission direction a are displayed on the display device 58.
The same operation is repeated for the emission directions b, c and so on, and blood flow images of all emission directions (flow velocity distribution images) are displayed on the display device 58. The flow velocity distribution images are displayed in color mode by being superimposed on a monochrome tomographic image.
However, the foregoing conventional ultrasonic Doppler imaging apparatus, which is based on the quadrature phase detection circuit 53 of analog circuit configuration, is deficient in the difficulty in the accurate balancing of the gain and phase of the I and Q channels, and therefore the accuracy of frequency analysis can be deteriorated due to unbalanced circuit parameters of both channels.